Lots of friction and heat there, I'm sure the chunks flying off the tire caught and then it crept back to the wheel. Maybe he was down to the belt by then too and made some sparks? Looks like it was a burnout contest or something, probably was some bleach or other lube used.
Actually, it'* pretty hard to get a tire lit. What they did there was run the ires all the way down to the cords. A regular tire would blow at this point, but the heavy cords on those off-road tires just got red-hot from friction, and that'* what caught the rubber on fire.